At licensed spectrum, there is only one owner of that spectrum in an area, which facilitates to create a single depot of information for base stations (eNBs). For example, eNBs under one operator can exchange channel state information and scheduling information. With single cell scheduling, radio resource access is typically controlled by eNB in an LTE system. In Coordinated Multipoint Operation (CoMP) and eCoMP, centralized or distributed scheduling can be used to coordinate the transmissions from eNBs with a direct goal to achieve either higher SINRs or interference mitigation. A salient point about (e)CoMP is identified as information exchange is through a network link which is either proprietary or standard based (e.g. X2). Typically, the exchanged information carried over those links need to happen with latency up to tens of milliseconds. In summary, interference handling is a central issue in wireless communications and the sole ownership of licensed spectrum has enabled information exchange among eNBs under one operator to achieve effective interference handling.
Third generation partnership project (3GPP) and Long Term Evolution (LTE) mobile telecommunication systems provide high data rate, lower latency and improved system performances. With the rapid development of “Internet of Things” (IOT) and other new user equipment (UE), the demand for supporting machine communications increases exponentially. To meet the demand of this exponential increase in communications, additional spectrum (i.e. radio frequency spectrum) is needed. The amount of licensed spectrum is limited. Therefore, communications providers need to look to unlicensed spectrum to meet the exponential increase in communication demand. One suggested solution is to use a combination of licensed spectrum and unlicensed spectrum. This solution is referred to as “Licensed Assisted Access” or “LAA”.
In LAA, an established communication protocol such as LTE can be used over the licensed spectrum to provide a first communication link, and LTE can also be used over the unlicensed spectrum to provide a second communication link. Furthermore, while LAA only utilizes the unlicensed spectrum to boost downlink through a process of carrier aggregation, enhanced LAA (eLAA) allows uplink streams to take advantage of the unlicensed band as well. The unlicensed band could be ISM band (Industrial Scientific Medical Band) at 2.4 GHz or 5 GHz, or it could be CBRS band (Citizens Broadband Radio Service band) at 3.5 GHz as long as no spectrum auction procedure takes place. Although eLAA is straightforward in theory, practical usage of eLAA while complying with various government regulations regarding the usage of unlicensed spectrum is not so straightforward. As no entity, network operator or otherwise, has a monopoly of using a certain frequency spectrum in an area, there can be wireless communication equipments outside the control of an operator which interfere with that operator's equipments. Furthermore, there is no single depot where information about channel states and traffic converge. As a result, the interference handling schemes developed at licensed spectrum such as (e)CoMP, (e)ICIC, etc., may no longer work at unlicensed spectrum. Hence, there is a need to coordinate the transmissions from equipments made by different eNB vendors, or allow collaboration among equipments made by different eNB vendors.
Listen-before-talk (LBT) schemes are discussed for solving the issue caused from the coexistence between WiFi and Licensed Assisted Access (LAA) and between LAA and LAA. To facilitate efficient and fair spectrum sharing, the dynamic spectrum sharing mechanism LBT need to be supported for both downlink LAA and uplink LAA based on regulation rules in each country. Though LTE applies request-and-grant-based uplink scheduling principle, UL LBT is still needed. In current LTE system, there are 4 subframe time difference between PDCCH (grant command from eNB) and PUSCH (uplink transmission by UE). UE has to perform UL LBT to confirm channel ownership. When more than one UEs contend uplink transmission for a given time slot in unlicensed band, uplink LBT scheme should perform in a proper way to reflect service prioritization.